install script assumes path of asdf

In plugins/nodejs/bin/install, the invocation of `asdf reshim` assumes that it is findable on the PATH, which may not always be the case.

`asdf reshim "$(plugin_name)" "$ASDF_INSTALL_VERSION"`

I was able to work around it by modifying ../lib/utils.sh to export ASDF_DIR:

```export ASDF_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")/../../.." && pwd)```

and prepending the path to `asdf` in install:

```$ASDF_DIR/bin/asdf reshim "$(plugin_name)" "$ASDF_INSTALL_VERSION"```

I will submit a patch for this if you like.