Require "phpstan/phpstan" for development and add to the Travis CI
configuration file to execute the analysis when Travis executes the
build with the version 7.2 of PHP.
The level of the configuration is very week for now (just "1") and still
quite some changes had to be made in order to make the analysis pass. I
hope it does not take much time to increase the level of the strictness
of the analyses.
I tried to configure that before but because of dependencies with
"symfony/validator" it was not possible.
Signed-off-by: Henrique Moody <henriquemoody@gmail.com>