make tests running on both java 7 and java 8 environments

2 jobs for master in 11 minutes and 39 seconds (queued for 7 seconds)
Status Job ID Name Coverage
  Build
failed #443
java7

00:00:24

passed #442
java8

00:11:15

 
Name Stage Failure
failed
java7 Build There is an unknown failure, please try again
11139 [ERROR] 
11139 [ERROR] Re-run Maven using the -X switch to enable full debug logging.
11140 [ERROR]
11140 [ERROR] For more information about the errors and possible solutions, please read the following articles:
11140 [ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/PluginExecutionException
11140 [ERROR]
11140 [ERROR] After correcting the problems, you can resume the build with the command
11141 [ERROR] mvn <goals> -rf :org.ow2.sat4j.core
ERROR: Job failed: exit code 1