Add various enhancements to the Travis script
Add various enhancements to the Travis script

* Add tests against supported LTS versions of Symfony
* Add a dedicated build for style checking

theofidry/DunglasApiBundle,teohhanhui/api-platform-core,vincentchalamon/core,api-platform/core,soyuka/DunglasApiBundle,api-platform/core,meyerbaptiste/core,soyuka/core,vincentchalamon/core,soyuka/core,teohhanhui/api-platform-core,meyerbaptiste/core,teohhanhui/api-platform-core,api-platform/core,soyuka/core,vincentchalamon/core,meyerbaptiste/core,vincentchalamon/core,jdeniau/DunglasApiBundle,jdeniau/DunglasApiBundle,jdeniau/DunglasApiBundle,meyerbaptiste/core,leesiongchan/DunglasApiBundle,soyuka/core,jdeniau/DunglasApiBundle,api-platform/core,teohhanhui/api-platform-core,teohhanhui/DunglasApiBundle,ahlee2326/DunglasApiBundle