Fixes #377 - adds check for global GA object

Global Google Analytics object wasn’t being initialized outside of
production, which caused an error that stopped other JS modules from
loading. This commit adds a check in the google-analytics-manager
script to check for the existence of the _gaq global object, before
doing initialization of Google Analytics settings.
Merge pull request #380 from codeforamerica/377-ga-fix

Fixes #377 - adds check for global GA object