MDL-52472 travis: enable php7

Relegate 5.6 runs (following the principle of highest and lowest version
only, to conserve travis resources and get faster test completions)

.travis.yml

@@ -12,16 +12,12 @@ notifications:
 language: php
 
 php:
-    # Moodle supports versions 5.4, 5.5, and 5.6 of PHP.
-    # Order by fastest to slowest.
-    # We currently only run the highest and lowest versions to reduce the load on travis-ci.org.
-    - 5.6
+    # We only run the highest and lowest supported versions to reduce the load on travis-ci.org.
+    - 7.0
+    # - 5.6
     # - 5.5
     - 5.4
 
-    # We hope to offer PHP 7 support in the near future.
-    - nightly
-
 services:
     # Ensure that memcached and mongodb are running for testing of those MUC stores.
     - memcached
@@ -50,29 +46,18 @@ matrix:
     # It will not stop the jobs from running.
     fast_finish: true
 
-    # Always allow failure on nightly.
-    # It's a nightly build and failures can happen.
-    allow_failures:
-        - php: nightly
-
     exclude:
-        # PHP 7 is not yet supported for actual runs.
-        # Exclude it by default - we include it for CITEST only later.
-        - php: nightly
-
         # MySQL - it's just too slow.
-        # Exclude it on all versions except for 5.6.
+        # Exclude it on all versions except for 7.0
+        # - env: DB=mysqli   PHPUNIT=true    INSTALL=false   CITEST=false
+        #   php: 5.6
+        #
         # - env: DB=mysqli   PHPUNIT=true    INSTALL=false   CITEST=false
         #   php: 5.5
 
         - env: DB=mysqli   PHPUNIT=true    INSTALL=false   CITEST=false
           php: 5.4
 
-    include:
-        # Attempt to run the CITEST set on PHP 7.
-        - php: nightly
-          env: DB=none     PHPUNIT=false   INSTALL=false   CITEST=true
-
 cache:
     directories:
       - $HOME/.composer/cache