Fix '--version' flag when not running from git checkout.

The version of tfenv itself was hardcoded. When not running from
a git checkout this needs to be manually updated. Instead, it now
looks at the Changelog to find the latest released version of its
running TFENV_ROOT.

CHANGELOG.md

@@ -1,3 +1,7 @@
+## 1.0.1 (unreleased)
+
+ * Fix '--version' flag to return version from CHANGELOG.md when not running from a git checkout.
+
 ## 1.0.0 (June 9, 2019)
 
  * A number of bugfixes and basic script improvements

libexec/tfenv---version

@@ -12,7 +12,7 @@
 set -e
 [ -n "${TFENV_DEBUG}" ] && set -x
 
-version="0.6.0"
+version=$(awk '/^##/{ print $2; exit}' "${TFENV_ROOT}"/CHANGELOG.md)
 git_revision=""
 
 if cd "${BASH_SOURCE%/*}" 2>/dev/null && git remote -v 2>/dev/null | grep -q tfenv; then