Update to Java 7

[mkordas]

Currently Travis build does not work for Java 6 on Travis (log from master branch), as Qulice requires at least Java 7:

Caused by: java.lang.UnsupportedClassVersionError: com/puppycrawl/tools/checkstyle/api/CheckstyleException : Unsupported major.minor version 51.0
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClass(ClassLoader.java:643)
    at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
    at java.net.URLClassLoader.defineClass(URLClassLoader.java:277)
    at java.net.URLClassLoader.access$000(URLClassLoader.java:73)
    at java.net.URLClassLoader$1.run(URLClassLoader.java:212)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:205)
    at org.codehaus.plexus.classworlds.realm.ClassRealm.loadClassFromSelf(ClassRealm.java:401)
    at org.codehaus.plexus.classworlds.strategy.SelfFirstStrategy.loadClass(SelfFirstStrategy.java:42)
    at org.codehaus.plexus.classworlds.realm.ClassRealm.unsynchronizedLoadClass(ClassRealm.java:271)
    at org.codehaus.plexus.classworlds.realm.ClassRealm.loadClass(ClassRealm.java:254)
    at org.codehaus.plexus.classworlds.realm.ClassRealm.loadClass(ClassRealm.java:239)
    at com.qulice.maven.DefaultValidatorsProvider.external(DefaultValidatorsProvider.java:66)
    at com.qulice.maven.CheckMojo.run(CheckMojo.java:82)
    at com.qulice.maven.CheckMojo.doExecute(CheckMojo.java:59)
    at com.qulice.maven.AbstractQuliceMojo.execute(AbstractQuliceMojo.java:175)
    at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:132)
    ... 20 more

Let's update project to Java 7 and remove failing Travis build.

Moreover, let's do typical Java 6 - > Java 7 migration steps: use diamond operator, try-with-resources, etc.

[dmarkov]

@yegor256 dispatch this issue please, see par.21

[dmarkov]

@mkordas thanks for the ticket, your account was topped for 15 mins, payment 56c32b5753fc494c8a0002c4

[dmarkov]

@nesterov-n can you please help? Keep in mind this. If you have any technical questions, don't hesitate to ask right here

Budget here is 30 mins (keep this principle in mind)

[nesterov-n]

@dmarkov See my PR #38

[dmarkov]

@dmarkov See my PR #38

@nesterov-n thank you

[nesterov-n]

@mkordas I believe that all is done here. Can you close issue?

[dmarkov]

@nesterov-n Many thanks! 30 mins were added to your account in Transaction ID AP-7HR88999GH051831E (task took 167 hours and 31 mins); +30 added to your rating, current score is: -28