Support Maven 3.X

[TheSnoozer]

This Refs to #316
Local tests with

#!/bin/bash
for mavenBasePath in /usr/share/maven-3*
do
  echo "============================================"
  ${mavenBasePath}/bin/mvn --version
  echo ""
  # ${file}/bin/mvn clean initialize -Pdemo
  for pluginVersion in 2.2.4-SNAPSHOT
  do
    rm -rf target
    # echo ${pluginVersion}
    echo "${mavenBasePath}/bin/mvn pl.project13.maven:git-commit-id-plugin:${pluginVersion}:revision"
    ${mavenBasePath}/bin/mvn pl.project13.maven:git-commit-id-plugin:${pluginVersion}:revision -Pdemo > /dev/null
    rc=$?;
    if [[ $rc != 0 ]]; then
      echo "pl.project13.maven:git-commit-id-plugin:${pluginVersion} is NOT compatible";
    else
      echo "pl.project13.maven:git-commit-id-plugin:${pluginVersion} is compatible"
    fi

    echo ""
    cat target/testing.properties | sed 's/\(.*\)/  \1/'
  done
done

didn't yield any execution problems.
However for older Maven-Versions (3.0.X) there is a warning from SLF4J:

[INFO] --- git-commit-id-plugin:2.2.4-SNAPSHOT:revision (get-the-git-infos) @ git-commit-id-plugin ---
[INFO] dotGitDirectory /home/stefan/workspace/maven-git-commit-id-plugin/.git
SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder".
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See http://www.slf4j.org/codes.html#StaticLoggerBinder for further details.

This does not seem to impact the execution of the plugin and thus will be a known issue.

Also this updates the travis-build in a way that

• java 7, 8, 9 is being tested (as previously)
• plugin is being build with maven versions 3.0, 3.1.1, 3.2.5, 3.3.9 and 3.5.0

I would not add tests for any other minor versions to keep the builds to a somewhat minimum.

[TheSnoozer]

Thinking more about what I just pushed.
The different builds with Maven might not be what I want to test with travis.
i want to verify if the build plugin can be executed within travis under certain maven projects.
Currently it only tests if the project can be build with different maven versions (which should always be the case).

Tests would needed to be adjusted to reflect that.
Don't merge this yet.

[TheSnoozer]

Just as an interesting side-note:
I did some testing and everything seems to behave as expected/wanted.
I just stumbled over one think where I was wondering why the build passed with <maven-plugin-api.version>3.1.1</maven-plugin-api.version> and maven 3.0. This should technically exactly what this thing is doing and prevent the build. Well blame maven for it and it was fixed for 3.0.2 (see https://issues.apache.org/jira/browse/MNG-4840).