Fixed issue when building on mac os x with java 1.8

compile:
    [mkdir] Created dir: /Users/manuel/git/jicofo/classes
    [javac] Compiling 59 source files to /Users/manuel/git/jicofo/classes
    [javac] warning: [options] bootstrap class path not set in conjunction with -source 1.5
    [javac] warning: [options] source value 1.5 is obsolete and will be removed in a future release
    [javac] warning: [options] target value 1.5 is obsolete and will be removed in a future release
    [javac] warning: [options] To suppress warnings about obsolete options, use -Xlint:-options.
    [javac] /Users/manuel/git/jicofo/src/org/jitsi/jicofo/JitsiMeetConference.java:1860: error: reference to Base64 is ambiguous
    [javac]             byte[] messageBytes = Base64.decode(messageBase64);
    [javac]                                   ^
    [javac]   both class java.util.Base64 in java.util and class net.java.sip.communicator.util.Base64 in net.java.sip.communicator.util match
    [javac] Note: /Users/manuel/git/jicofo/src/org/jitsi/jicofo/ProtocolProviderHandler.java uses unchecked or unsafe operations.
    [javac] Note: Recompile with -Xlint:unchecked for details.
    [javac] 1 error
    [javac] 4 warnings