Bug 853041 - [Dialer] Solving Lint errors (follow-up of 845487) (r=fcampo)

Basically, we need to solve a couple of Lint errors detected by Travis:
----- FILE  :  /home/travis/build/mozilla-b2g/gaia/apps/communications/contacts/oauth2/js/oauth20.js -----
Line 84, E:0131: Single-quoted string preferred over double-quoted string.
Line 107, E:0131: Single-quoted string preferred over double-quoted string.
Found 2 errors, including 0 new errors, in 1 files (603 files OK).