Downgrade gothemis compatibility

[vixentael]

It seems that gothemis uses syntax incompatible with old Go versions (TODO: check exact version, but it's definitely below 1.2).

We should re-write this line to smth more compatible
https://github.com/cossacklabs/themis/blob/master/gothemis/session/session.go#L93

As example we used this code in Hermes:

//CArrayToSlice return buffer as byte slice with len/cap equals to bufferLength
func CArrayToSlice(buffer unsafe.Pointer, bufferLength int) []byte {
	sliceHeader := reflect.SliceHeader{uintptr(buffer), int(bufferLength), bufferLength}
	data := *(*[]byte)(unsafe.Pointer(&sliceHeader))
	return data
}

(from https://github.com/cossacklabs/hermes-core/blob/master/gohermes/utils.go)

Although this code is 100% valid, but it would be nice to support previous versions.

[vixentael]

Self-check:
Internal buildbot builds that are supposed to be fixed after this fix:

• themis-test-ubuntu_precise-x86_64
• themis-test-ubuntu_precise-i386 (go1)
• themis-test-debian_wheezy-i386

[vixentael]

closed by #254