Upgrade default go to 1.7.1.

This picks up bugfixes since the 1.7 release. Release notes are here:
  https://golang.org/doc/devel/release.html#go1.7.minor

Testing Done:
Locally green:
```
./pants test contrib/go/examples/::
./pants test contrib/go/tests/::
```

CI went green here:
  https://travis-ci.org/pantsbuild/pants/builds/158489923

Bugs closed: 3842

Reviewed at https://rbcommons.com/s/twitter/r/4210/

contrib/go/src/python/pants/contrib/go/subsystems/go_distribution.py

@@ -32,7 +32,7 @@ def register_options(cls, register):
       register('--supportdir', advanced=True, default='bin/go',
                help='Find the go distributions under this dir.  Used as part of the path to lookup '
                     'the distribution with --binary-util-baseurls and --pants-bootstrapdir')
-      register('--version', advanced=True, default='1.7',
+      register('--version', advanced=True, default='1.7.1',
                help='Go distribution version.  Used as part of the path to lookup the distribution '
                     'with --binary-util-baseurls and --pants-bootstrapdir')