main: rework "you need Go 1.5" lockout to make "go vet" happy

When you run "go vet" explicitely against go1.4.go, it ignores
the "+build !go1.5" tag and, of course, throws a syntax error:

  $ go vet go1.4.go
  can't load package: package main:
  go1.4.go:5:1: expected 'package', found 'STRING' "You need Go 1.5 or higher to compile gocryptfs!"

Unfortunatey, this is how https://goreportcard.com/ seems to call
"go vet", and means we get 0% on the "go vet" test and see this
error:

  An error occurred while running this test (strconv.Atoi: parsing " go1.4.go": invalid syntax)

By reworking the logic to use a non-existant package we get an
uglier error

  $ GOROOT=/opt/go1.4.3 /opt/go1.4.3/bin/go build
  go1.4.go:7:8: cannot find package "You_need_Go_1.5_or_higher_to_compile_gocryptfs" in any of:
  	/opt/go1.4.3/src/You_need_Go_1.5_or_higher_to_compile_gocryptfs (from $GOROOT)
  	/home/jakob/go/src/You_need_Go_1.5_or_higher_to_compile_gocryptfs (from $GOPATH)
  profiling.go:6:2: cannot find package "runtime/trace" in any of:
  	/opt/go1.4.3/src/runtime/trace (from $GOROOT)
  	/home/jakob/go/src/runtime/trace (from $GOPATH)

but make "go vet" happy.

Author: rfjakob

go1.4.go

@@ -1,5 +1,7 @@
 // +build !go1.5
 
+package main
+
 // Cause an early compile error on Go 1.4 an lower. We need Go 1.5 for a number
-// of reasons, among them NewGCMWithNonceSize and RawURLEncoding.
-"You need Go 1.5 or higher to compile gocryptfs!"
+// of reasons, among them NewGCMWithNonceSize, RawURLEncoding, runtime/trace.
+import "You_need_Go_1.5_or_higher_to_compile_gocryptfs"