A value is now deleted by calling: delete(@x)

Add ability to get kernel version from header at runtime

Useful if bpftrace is compiled on a different kernel version
than it's being run on.

They should be run with LLVM 5

BCC has added support for attaching kprobes to custom offsets from the
start of a function. Just use 0 for now.

Add Travis CI builds and tests

e.g. The first symbol would be blank/corrupted when running:
    printf("%s %s\n", func, func)

This was caused by a dodgy conversion from std::string
to C-string, where the result of ".c_str()" was being
saved, but the underlying std::string was modified by
the vector it was being stored in, invalidating the C-string

fix for #18: initializing comm memory

Static linking can be selected with -DSTATIC_LINKING:BOOL=ON