[AIRFLOW-520] Fix Version Info in Flask UI

[kaxil]

Make sure you have checked all steps below.

Jira

• My PR addresses the following Airflow Jira issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
  • https://issues.apache.org/jira/browse/AIRFLOW-520

Description

• Here are some details about my PR, including screenshots of any UI changes:
  

That is because of the following issue:

>>> import pkg_resources
>>> airflow_version = pkg_resources.require("apache-airflow")[0].version
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/kaxil/.virtualenvs/airflow_stable_pip/lib/python2.7/site-packages/pkg_resources/__init__.py", line 892, in require
    needed = self.resolve(parse_requirements(requirements))
  File "/Users/kaxil/.virtualenvs/airflow_stable_pip/lib/python2.7/site-packages/pkg_resources/__init__.py", line 783, in resolve
    raise VersionConflict(dist, req).with_context(dependent_req)
pkg_resources.ContextualVersionConflict: (Flask-Login 0.2.11 (/Users/kaxil/.virtualenvs/airflow_stable_pip/lib/python2.7/site-packages), Requirement.parse('Flask-Login<0.5,>=0.3'), set(['flask-appbuilder']))

This also brings it in line with RBAC FAB UI.

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

Commits

• My commits all reference Jira issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

• In case of new functionality, my PR adds documentation that describes how to use it.
  • When adding new operators/hooks/sensors, the autoclass documentation generation needs to be added.

Code Quality

• Passes flake8

[kaxil]

@ashb Can we include this in 1.10, please ??? 😉

[codecov-io]

Codecov Report

Merging #4072 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #4072      +/-   ##
==========================================
- Coverage   77.69%   77.68%   -0.01%     
==========================================
  Files         199      199              
  Lines       15957    15956       -1     
==========================================
- Hits        12397    12396       -1     
  Misses       3560     3560

Impacted Files	Coverage Δ
airflow/www/views.py	68.89% <0%> (-0.02%)	⬇️

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[ashb]

Do we need the try/except anymore?