Use specific commits from ruby-build. Allow Ruby 2.6.3 Installation

Addressing #108 and given ruby-build's reluctance to cut timely releases as
stated therein and from the perspective of ruby-build users, move asdf-ruby to
pulling specific commits from ruby-build.

lib/utils.sh

-RUBY_BUILD_VERSION="20190401"
-RUBY_BUILD_TAG="v$RUBY_BUILD_VERSION"
+RUBY_BUILD_VERSION="44b0c75"
+RUBY_BUILD_TAG="$RUBY_BUILD_VERSION"
 
 echoerr() {
   >&2 echo -e "\033[0;31m$1\033[0m"