Fix es6-module-transpiler version

`broccoli-es6-concatenator` can use only `es6-module-transpiler` with version < 0.5.0 because in ~0.5.0 `require('es6-module-transpiler').Compiler` has been removed.

See also https://github.com/square/es6-module-transpiler/compare/v0.4.0...v0.5.0#diff-04c6e90faac2675aa89e2176d2eec7d8L87