min-required correctly finds tagged release versions

[mrpaws]

Problem: 'tfenv <install|use> min-required' incorrectly interprets a tagged release (ie rc1,beta2,alpha3).

For example, ">=0.13.0-rc1" is mangled by the translation and results in interpreting min-required as "0.13.1". This is a bug because at worst results can install and use an undesired version, but mostly is annoying for testing tagged versions.

Solution
The solution is to use a basic regex with 'grep' instead of 'tr' to properly acquire both normal and tagged release versions. An extra test was added to account for this; the results are below.

Notes

• This makes 'tfenv min-required' usable with every release of terraform to date (https://releases.hashicorp.com/terraform/) - - 'grep' is used with basic regex instead of an extended regular expression (egrep/grep -E) to facilitate maximum cross-compatibility between platforms and varying implementations of grep.

Problem in practice (debug):

++ grep -h -R required_version --include '*tf' <FILES SCANNED>
++ tr -c -d '0-9. ~=!<>'
+ versions='     = >=0.13.01'
+ [[      = >=0.13.01 =~ ([~=!<>]{0,2}[[:blank:]]*[0-9]+[0-9.]+)[^0-9]* ]]
+ found_min_required='>=0.13.01'
+ [[ >=0.13.01 =~ ^!=.+ ]]
++ echo '>=0.13.01'
++ tr -c -d 0-9.
+ found_min_required=0.13.01
+ echo 0.13.01
+ exit 0
No installed versions of terraform matched 'min-required'

Solution in practice (debug):

++ grep -h -R required_version --include '*tf' <FILES SCANNED>
++ grep -o '\(\d\+\.\?\)\{2,3\}\(-[a-z]\+[0-9]\+\)\?'
+ versions=0.13.0-rc1
+ [[ 0.13.0-rc1 =~ ([~=!<>]{0,2}[[:blank:]]*[0-9]+[0-9.]+)[^0-9]*(-[a-z]+[0-9]+)? ]]
+ found_min_required=0.13.0-rc1
+ [[ 0.13.0-rc1 =~ ^!=.+ ]]
++ echo 0.13.0-rc1
+ found_min_required=0.13.0-rc1
+ echo 0.13.0-rc1
+ exit 0
Installing Terraform v0.13.0-rc1
Downloading release tarball from https://releases.hashicorp.com/terraform/0.13.0-rc1/terraform_0.13.0-rc1_darwin_amd64.zip
######################################################################## 100.0%
Downloading SHA hash file from https://releases.hashicorp.com/terraform/0.13.0-rc1/terraform_0.13.0-rc1_SHA256SUMS
No keybase install found, skipping OpenPGP signature verification
Archive:  tfenv_download.Lubb4P/terraform_0.13.0-rc1_darwin_amd64.zip
  inflating: tfenv/versions/0.13.0-rc1/terraform
Installation of terraform v0.13.0-rc1 successful. To make this your default version, run 'tfenv use 0.13.0-rc1

[mrpaws]

Reference Issue: #207 (comment)