fix the coveralls report

Summary:
Ever since the switch from `test.sh` to `test.py`, we've been generating
the `.coverage` file inside `tests/`. Update `.travis.yml` to run
`coveralls` in the right place.

Also, handle the `--with-coverage` flag in a slightly more flexible way.

Test Plan:
Watch the build in my fork and make sure it looks like it's going to
work.

Reviewers: sean

Reviewed By: sean

Differential Revision: https://phabricator.buildinspace.com/D182

.travis.yml

@@ -8,4 +8,4 @@ install:
 script:
   - ./test.py --with-coverage -v
 after_success:
-  coveralls
+  cd tests && coveralls

test.py

@@ -35,8 +35,8 @@ def main():
     env = os.environ.copy()
     env['PYTHONPATH'] = REPO_ROOT
     args = sys.argv[1:]
-    if len(args) > 0 and args[0] == '--with-coverage':
-        args.pop(0)
+    if len(args) > 0 and '--with-coverage' in args:
+        args.remove('--with-coverage')
         command_start = ['coverage', 'run']
     else:
         command_start = [sys.executable]