Fix: Spidermonkey dropped --utf8 flag

Four months ago, Spidermonkey dropped the `-U` (`--utf8`) flag (see https://github.com/mozilla/mozilla-central/commit/53c97c#L16L4876). That change breaks (*) amo-validator when the latest version of Spidermonkey is installed. This proposal fixes the bug while maintaining backwards-compatible.

If you want to completely drop support for old Spidermonkey versions, removing `'-U'` from line 170 also fixes the bug.

(*) "breaks" = amo-validator crashes with a simplejson.decoder.JSONDecodeError, because `data` is not valid JSON. In fact, `data` holds the same value as `js --help` (Spidermonkey usage).
Bug 833512 - Flag instances of null passed in place of a privacy context
Merge pull request #195 from kmaglione/bug-833512

Bug 833512 - Flag instances of null passed in place of a privacy context
remove duplicate test
Flag global private browsing service (bug 826079)
Flag decodeImageData for Fx20 (bug 852679)
Flag DOMNSEditable (bug 852668)
clean up deprication warning and private browsing code
Flag nsIProfile (bug 852663)
Flag places import export (bug 852654)
Updated amo-validator with hashes for SDK version 1.14
Merge pull request #196 from KWierso/1.14

Updated amo-validator with hashes for SDK version 1.14
Fix for CDATA elements getting munched (bug 861522)
Include Py2.7.4 copy module for 2.6 compat (bug 853547)
Merge pull request #194 from Rob--W/patch-1

Fix: Spidermonkey dropped --utf8 flag
Added compatibility tests for Thunderbird 18 (Bug 850029)
Merge pull request #197 from wagnerand/master

Added compatibility tests for Thunderbird 18 (Bug 850029)
Added compatibility tests for Thunderbird 19 (Bug 851922)
Added compatibility tests for Thunderbird 20 (Bug 867956)
Merge pull request #198 from wagnerand/master

 Added compatibility tests for Thunderbird 19 (Bug 851922)