[CI] Add PHP 7.2 to travis build matrix (#10)

* [CI] Add PHP 7.2 to travis build matrix

Since PHP 7.2 will be soon out of the oven we should start testing
against 7.2 and support it.

* [COMPOSER] Bump `phpstan/phpstan` to version 0.8

* [COMPOSER] Bump `guzzlehttp/guzzle` to version 6.3

In order to support PHP 7.2 we need to bump the version, otherwise the
builds will fail.

See https://travis-ci.org/flix-tech/schema-registry-php-client/jobs/282192876
for more details.

Author: tPl0ch

.travis.yml

@@ -8,6 +8,7 @@ services:
 php:
   - '7.0'
   - '7.1'
+  - '7.2'
   - nightly
 
 env:

composer.json

@@ -12,14 +12,14 @@
   ],
   "require": {
     "php": "~7.0",
-    "guzzlehttp/guzzle": "~6.2",
+    "guzzlehttp/guzzle": "~6.3",
     "beberlei/assert": "~2.7",
     "rg/avro-php": "~1.8"
   },
   "require-dev": {
     "phpunit/phpunit": "~6.2",
     "friendsofphp/php-cs-fixer": "~2.0",
-    "phpstan/phpstan": "~0.7",
+    "phpstan/phpstan": "~0.8",
     "raphhh/trex-reflection": "~1.0",
     "doctrine/cache": "~1.3"
   },