Updated Travis CI configuration - added PHP 7.3 to matrix configuration

Removed nightly, as it is no point to run this build right now.
Removed also 'sudo: false' as it is deprecated and no longer needed.

.travis.yml

@@ -1,5 +1,3 @@
-sudo: false
-
 language: php
 
 cache:
@@ -33,17 +31,15 @@ matrix:
     - php: 7.2
       env:
         - DEPS=latest
-    - php: nightly
+    - php: 7.3
       env:
       - DEPS=lowest
-    - php: nightly
+    - php: 7.3
       env:
       - DEPS=locked
-    - php: nightly
+    - php: 7.3
       env:
       - DEPS=latest
-  allow_failures:
-    - php: nightly
 
 before_install:
   - if [[ $TEST_COVERAGE != 'true' ]]; then phpenv config-rm xdebug.ini || return 0 ; fi