travis: xcode7.1 is retired, switch to xcode6.4

[guitorri]

No description provided.

[guitorri]

For some reason the example DC_AC_SP_stab_prj in qucs-test is now failing on OSX (it passes on Linux). The error message is not very helpful...
https://travis-ci.org/Qucs/qucs/jobs/179901782#L3867
https://travis-ci.org/Qucs/qucs/jobs/179901782#L4058

My Mac is being repaired and I cannot investigate the issue.

I will blacklist this test on OSX and move on.
We still have to investigate the numerical differences across platforms, so we can take care of it along with #205

[guitorri]

Now another test for the equation system is failing on OSX:
https://travis-ci.org/Qucs/qucs/jobs/180066353#L4471-L4491

This things were fine just before Travis changed the availble OSX images... I don't now if this is OS version dependent of is side effect of the ongoing issues Travis is having on their infrastructure.
There is a huge backlog of builds, every commit will take many hours to even start.... I am inclined to merge them as long as Linux builds are passing.

[guitorri]

I rerun the build and another project example is failing DC_AC_bbv_prj:
https://travis-ci.org/Qucs/qucs/jobs/180066353#L3891-L3898

I don't know what is going on. The run.py script seems to be randomly failing to run the commands in a subprocess.

[guitorri]

On the flip-side xcode6.4 does seem build the code.... I will merge and ignore the OSX testing issues for now.