Test against PHPUnit-5.1

PHPUnit-5.1 was released. CI did test against the 5.1 branch. Now it can
test against the release.

Author: malkusch

.travis.yml

@@ -9,7 +9,7 @@ cache:
 env:
   - PHPUNIT_VERSION=dev-master
   - PHPUNIT_VERSION=~5.2.0-dev
-  - PHPUNIT_VERSION=~5.1.0-dev
+  - PHPUNIT_VERSION=~5.1.0
   - PHPUNIT_VERSION=~5.0.0
   - PHPUNIT_VERSION=~4.8.0
   - PHPUNIT_VERSION=~4.7.0
@@ -35,7 +35,7 @@ matrix:
   exclude:
     - env: PHPUNIT_VERSION=~5.0.0
       php: 5.5
-    - env: PHPUNIT_VERSION=~5.1.0-dev
+    - env: PHPUNIT_VERSION=~5.1.0
       php: 5.5
     - env: PHPUNIT_VERSION=~5.2.0-dev
       php: 5.5