update VERSION for master
update VERSION for master

Summary: master suffix for master branch

$ mcrouter --version
mcrouter 9.0.0-master

will not affect our tagged release and/or release branch

Test Plan: build & run

Reviewed By: pavlo-fb

Differential Revision: D2510307

fb-gh-sync-id: c1c63440a2649693e637708d04e3aa3f6c310046

yqzhang/mcrouter,facebook/mcrouter,nvaller/mcrouter,reddit/mcrouter,yqzhang/mcrouter,nvaller/mcrouter,synecdoche/mcrouter,nvaller/mcrouter,synecdoche/mcrouter,facebook/mcrouter,nvaller/mcrouter,facebook/mcrouter,reddit/mcrouter,yqzhang/mcrouter,facebook/mcrouter,synecdoche/mcrouter,reddit/mcrouter,synecdoche/mcrouter,yqzhang/mcrouter,reddit/mcrouter