Update SnakeYAML dependency from 1.23 to 1.26

[josephearl]

Fixes #106

I had to replace mavenCentral() with the specific maven URL because it seems mavenCentral() on Gradle 2.0 uses the HTTP endpoint and that is no longer supported (returns a 501).

In addition one of the tests is failing for me:

io.github.azagniotov.stubby4j.StubsPortalTest > shouldReturnReplacedValueInLocationHeaderWhenQueryParamHasDynamicToken FAILED
    java.lang.AssertionError: 
    Expected: (an instance of java.net.UnknownHostException and exception with message a string containing "alex.com")
         but: an instance of java.net.UnknownHostException <java.net.ConnectException: Connection refused (Connection refused)> is a java.net.ConnectException

This is possibly due to a change in a Java version? (I was testing with Java 8)