Lore assumes pyenv is installed in a specific way

Specifically it assumes it is NOT installed through brew, but manually installed by cloning the git repo. If you try to run `lore install` with pyenv installed this way you get

```
> lore install
WARNING pyenv executable is not present at /Users/thomasfulton/.pyenv/bin/pyenv
Would you like to blow away ~/.pyenv and rebuild from scratch? [Y/n] n
ERROR please fix pyenv before continuing
```

If you symlink your pyenv executable to that location you eventually get

```
fatal: not a git repository (or any of the parent directories): .git
Traceback (most recent call last):
File "/Users/thomasfulton/.pyenv/versions/3.7.2/bin/lore", line 11, in
sys.exit(main())
File "/Users/thomasfulton/.pyenv/versions/3.7.2/lib/python3.7/site-packages/lore/__main__.py", line 355, in main
known.func(known, unknown)
File "/Users/thomasfulton/.pyenv/versions/3.7.2/lib/python3.7/site-packages/lore/__main__.py", line 652, in install
install_python_version()
File "/Users/thomasfulton/.pyenv/versions/3.7.2/lib/python3.7/site-packages/lore/__main__.py", line 1098, in install_python_version
subprocess.check_call(('git', '-C', env.PYENV, 'pull'))
File "/Users/thomasfulton/.pyenv/versions/3.7.2/lib/python3.7/subprocess.py", line 347, in check_call
raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '('git', '-C', '/Users/thomasfulton/.pyenv', 'pull')' returned non-zero exit status 128.
```