Re-establish no nailguns under TravisCI.

We used to have a switch for this in the ci.sh script that .travis.yml
passed but that went away here: https://rbcommons.com/s/twitter/r/1852/

This just adds a config override file dedicated to TravisCI runs.

Testing Done:
CI went green here:
  https://travis-ci.org/pantsbuild/pants/builds/58106974

Bugs closed: 1405

Reviewed at https://rbcommons.com/s/twitter/r/2065/

.travis.yml

@@ -23,6 +23,7 @@ install:
 env:
   global:
     - CXX=g++
+    - PANTS_CONFIG_OVERRIDE=pants.travis-ci.ini
     # Credentials for OSX syncing: GH_USER, GH_EMAIL, GH_TOKEN
     # These are encrypted with a public key for our repo that only
     # Travis-CI has the private key for.  We are trusting Travis-CI

pants.travis-ci.ini

@@ -0,0 +1,5 @@
+# Overrides for TravisCI runs.
+[DEFAULT]
+
+# Turn off all nailgun use.
+use_nailgun: False