Bump OkHttp to v3.9.0.

In addition to v3.0.1 being super old, it looks like earlier version(s)
of OkHttp have a race that will leak resources when requests are
enqueued instead of executed and cancel is called.

Progress towards #1996.

gradle.properties

@@ -17,7 +17,7 @@ POM_DEVELOPER_NAME=Sam Judd
 POM_DEVELOPER_EMAIL=judds@google.com
 ANDROID_SUPPORT_VERSION=26.0.2
 VOLLEY_VERSION=1.0.0
-OK_HTTP_VERSION=3.0.1
+OK_HTTP_VERSION=3.9.0
 ANDROID_GRADLE_VERSION=2.3.0
 
 COVERALLS_GRADLE_VERSION=2.4.0