Fixed the --excludes option

Using the --excludes option produced the following error when pushing translations:

java.lang.UnsupportedOperationException
	at java.util.AbstractList.add(AbstractList.java:148)
	at java.util.AbstractList.add(AbstractList.java:108)
	at org.zanata.client.commands.push.AbstractPushStrategy.addExcludesForLocaleFilenames(AbstractPushStrategy.java:112)
	at org.zanata.client.commands.push.AbstractPushStrategy.getSrcFiles(AbstractPushStrategy.java:98)
	at org.zanata.client.commands.push.PropertiesStrategy.findDocNames(PropertiesStrategy.java:80)
	at org.zanata.client.commands.push.PushCommand.pushCurrentModule(PushCommand.java:280)
	at org.zanata.client.commands.push.PushCommand.run(PushCommand.java:203)
	at org.zanata.client.commands.ConfigurableCommand.runWithActions(ConfigurableCommand.java:94)
	at org.zanata.client.commands.ArgsUtil.runCommand(ArgsUtil.java:48)
	at org.zanata.client.ZanataClient.processArgs(ZanataClient.java:168)
	at org.zanata.client.ZanataClient.main(ZanataClient.java:93)