travis: add a build using WolfSSL

[bagder]

No description provided.

[bagder]

@wolfSSL: shouldn't ./wolfssl/wolfcrypt/fips.h be present in the tarball of the source archive we can download from github?

Right now we get this build error when trying.

What's most odd perhaps is that in the "official" 3.14.0 download, that file is zero bytes:

$ ls -l wolfssl/wolfcrypt/fips*
-rw-r--r-- 1 daniel daniel    0 mar  2 22:40 wolfssl/wolfcrypt/fips.h
-rw-r--r-- 1 daniel daniel 1636 mar  2 22:38 wolfssl/wolfcrypt/fips_test.h

[bagder]

I've solved the merge conflict locally. This PR awaits a release from wolfssl with a fix for test 311.

Test 313 also fails, but that's testing CRL which we don't seem to have supported in the wolfssl backend!

[bagder]

Here's a version that explicitly switches off test 311 and 313...