perf: skip aliases

[aymericbouzy]

When this plugin is used in combination with nodenv-aliases, some aliases are created that match the regexp ^[0-9.]+$ and are therefore passed on to sh-semver which seems particularily slow at dealing with them.

Since aliases are redundant anyway, we can simply ask nodenv to skip them when listing installed node versions.

Fixes #8

Note

Tests fail on my machine for master branch, so I'm not able to tell if this change introduces a regression / not able to add a test case to cover this change.

I can confirm though that node -v is now snappy on my machine with that change.