Add exceptions for node@* versioned formulae to use conflicts_with
instead of keg_only :versioned_formula since they, and the main node
formula, all currently overwrite npm during postinstall.

The system Subversion doesn't handle new certificate authorities (e.g.
Let's Encrypt) well enough for this check to be useful.

Many parts of Homebrew assume that a version string beginning with
"HEAD" is, in fact, a head build. A stable version that begins with
"HEAD" violates this assumption and causes problems, as it's treated
as a head build in some places and as a stable build in others.

If it's not there, error out rather than fetching to avoid creating
versionless cache files, which cause `brew cleanup` to crash.

This smooths over the transition for users who have an existing
bundle config in this location due to having gems installed at the old
path.

Closes #2203.

Signed-off-by: Misty De Meo <mistydemeo@gmail.com>

Closes #2198.

Signed-off-by: Misty De Meo <mistydemeo@gmail.com>