Upgrade to Surefire 2.18

Notably this addresses SUREFIRE-1028 which prevented running single
test methods.

Changelog:

http://mail-archives.apache.org/mod_mbox/maven-announce/201411.mbox/%3CCAK2FUK2o_9oAUcWkAC=pbA5vejMKW0i0Pn9XBBo9cTg2uRFRYg@mail.gmail.com%3E

pom.xml

@@ -113,7 +113,7 @@
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-surefire-plugin</artifactId>
-        <version>2.15</version>
+        <version>2.18</version>
         <configuration>
           <parallel>classes</parallel>
           <threadCount>1</threadCount>