add rand builtin

See https://github.com/ajor/bpftrace/issues/47

Removes dependency on glibc for loading dynamic libraries at runtime
e.g. as in EngineBuilder::create()

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