Add note on Gradle requirements

[etlweather]

Gradle prior to 4.5 can't pull the dependencies when running through a proxy like Nexus.

[gmessner]

This should be moved below the "Gradle: build.gradle" section as follows:

Gradle: build.gradle

dependencies {
    ...
    compile group: 'org.gitlab4j', name: 'gitlab4j-api', version: '4.8.46'
}

NOTE: Pulling dependencies may fail when using Gradle prior to 4.5. See Gradle issue 3065

[etlweather]

Good point. Fixed.

[gmessner]

@etlweather
Thanks, the changes look good, merging.