Security vulnerabilities in dependencies

[shanethehat]

The version of Monolog used has a security vulnerability: Seldaek/monolog#448. The solution seems to be to upgrade to ~1.16.0.

The version of zend-http used has a security vulnerability: http://framework.zend.com/security/advisory/ZF2015-04. The solution to this is to upgrade to ~2.4.1.

[piotrekkaminski]

@shanethehat we have updated the packages and introduced Symfony dependency checker into our CI process to make sure we stay up to date on dependencies in the future.