travis: try to fix the build on emscripten
travis: try to fix the build on emscripten

The emsdk-portable .tar.gz now extracts to emsdk-portable instead of
emsdk_portable. Handle that.

graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust