Update Makefile and install script

fzf may not run correctly on some OS even when the binary the platform
is successfully downloaded. The install script is updated to check if
the system has no problem running the executable and fall back to Ruby
version when necessary.

src/Makefile

@@ -44,8 +44,8 @@ clean:
 docker:
 	docker build -t junegunn/ubuntu-sandbox .
 
-linux64:
+linux64: docker
 	docker run -i -t -u jg -v $(shell cd ..; pwd):/fzf junegunn/ubuntu-sandbox \
 		/bin/bash -ci 'cd ~jg/go/src/github.com/junegunn/fzf/src; make'
 
-.PHONY: build release install linux64 clean docker run
+.PHONY: build release install linux64 clean docker