Require PHP7 to pass.

Require PHP7 tests to pass, and allow coverage generation to fail
without breaking the build.

PHP7 has been pretty stable lately, while coverage has not. Switch the
two around to keep builds passing more often.

.travis.yml

@@ -37,7 +37,7 @@ matrix:
       env: HHVM=1 DB=mysql db_dsn='mysql://travis@0.0.0.0/cakephp_test'
 
   allow_failures:
-    - php: 7.0
+    - env: COVERALLS=1 DEFAULT=0
 
     - php: hhvm