Specify maven-compiler-plugin version to eliminate warning from Maven 3.

Warning from Maven 3:

[WARNING] Some problems were encountered while building the effective model for com.saucelabs:ci-sauce:jar:1.70-SNAPSHOT
[WARNING] 'build.plugins.plugin.version' for org.apache.maven.plugins:maven-compiler-plugin is missing. @ line 52, column 21
[WARNING]
[WARNING] It is highly recommended to fix these problems because they threaten the stability of your build.
[WARNING]
[WARNING] For this reason, future Maven versions might no longer support building such malformed projects.