Updated build.gradle to use the latest version of Volley from the mirror at https://github.com/mcxiaoke/android-volley
new test reproducing: #1549
fix testVolleyImageLoaderGetNullHost to check for a null host, not a null url
Merge pull request #1555 from wordpress-mobile/issue/1549-npe-network-imageview

issue/1549-npe-network-imageview