Add TravisCi integration and update build tools (#21)

This fixes #19

pom.xml

@@ -21,12 +21,12 @@
         visit https://github.com/groovy/GMavenPlus/wiki -->
         <groupId>org.codehaus.gmavenplus</groupId>
         <artifactId>gmavenplus-plugin</artifactId>
-        <version>1.5</version>
+        <version>1.6</version>
         <executions>
           <execution>
             <goals>
               <goal>compile</goal>
-              <goal>testCompile</goal>
+              <goal>compileTests</goal>
             </goals>
           </execution>
         </executions>
@@ -35,7 +35,7 @@
       <!-- Only required if names of spec classes don't match default Surefire patterns (`*Test` etc.) -->
       <plugin>
         <artifactId>maven-surefire-plugin</artifactId>
-        <version>2.18.1</version>
+        <version>2.20.1</version>
         <configuration>
           <useFile>false</useFile>
           <includes>
@@ -66,7 +66,7 @@
     <dependency> <!-- use a specific Groovy version rather than the one specified by spock-core -->
       <groupId>org.codehaus.groovy</groupId>
       <artifactId>groovy-all</artifactId>
-      <version>2.4.10</version>
+      <version>2.4.13</version>
     </dependency>
     <dependency> <!-- enables mocking of classes (in addition to interfaces) -->
       <groupId>net.bytebuddy</groupId>