always build multicall binary

squashed:
	a2c6b27 - build: automatically generate main() files
	c942f0f - remove MULTICALL=1 build from travis
	cb7b35b - make: remove unnecessary shell command
	69bbb31 - update README
	03a3168 - all: move main() into separate file that links against util crate
	8276384 - make: always build multicall binary
	aa4edeb - make: avoid 'rustc --crate-file-name'
Merge pull request #309 from ebfe/build

make: always build multicall binary