Update the README.md to use the up to date build command.
Update the README.md to use the up to date build command.

This is the command used by the travis build and the runtests.py script.

incrediblesound/spec,AndrewScheidecker/WebAssembly-spec,binji/spec,binji/spec,kg/spec,naturaltransformation/spec,AugustusHuang/spec,maurer/spec,lindig/spec,pjuftring/spec,kg/spec,AndrewScheidecker/WebAssembly-spec,lindig/spec,naturaltransformation/spec,marianoguerra/spec,Acidburn0zzz/spec,maurer/spec,AugustusHuang/spec,pjuftring/spec,marianoguerra/spec,Acidburn0zzz/spec,incrediblesound/spec