[AIRFLOW-2960] Pin boto3 to <1.8 (#3810)

Boto 1.8 has been released a few days ago and they break our tests.

Author: Fokko

setup.py

@@ -181,7 +181,7 @@ def write_version(filename=os.path.join(*['airflow',
     'elasticsearch>=5.0.0,<6.0.0',
     'elasticsearch-dsl>=5.0.0,<6.0.0'
 ]
-emr = ['boto3>=1.0.0']
+emr = ['boto3>=1.0.0, <1.8.0']
 gcp_api = [
     'httplib2>=0.9.2',
     'google-api-python-client>=1.6.0, <2.0.0dev',
@@ -219,7 +219,7 @@ def write_version(filename=os.path.join(*['airflow',
 qds = ['qds-sdk>=1.9.6']
 rabbitmq = ['librabbitmq>=1.6.1']
 redis = ['redis>=2.10.5']
-s3 = ['boto3>=1.7.0']
+s3 = ['boto3>=1.7.0, <1.8.0']
 salesforce = ['simple-salesforce>=0.72']
 samba = ['pysmbclient>=0.1.3']
 segment = ['analytics-python>=1.2.9']