Bash 3.x compatibility

[bayandin]

Default bash installed on macOS is 3.2.57.
And there are some difference in brace expansion for @ for bash 3.x and 5.x:

$ /bin/bash --version
GNU bash, version 3.2.57(1)-release (x86_64-apple-darwin19)
Copyright (C) 2007 Free Software Foundation, Inc.

$ /bin/bash -c 'set -u; echo "${@}OK"'
/bin/bash: @: unbound variable

$ /usr/local/bin/bash --version
GNU bash, version 5.0.17(1)-release (x86_64-apple-darwin19.4.0)
Copyright (C) 2019 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>

This is free software; you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

$ /usr/local/bin/bash -c 'set -u; echo "${@}OK"'
OK

I suggest to replace ${@} with $@ to make tfenv 2 compatible with default bash on macOS:

$ /bin/bash -c 'set -u; echo "$@OK"'
OK

$ /usr/local/bin/bash -c 'set -u; echo "$@OK"'
OK

I ran shellcheck against my changes and it suggested to replace $@ to $* for printing logs, since it was mixing string variables and arrays (https://github.com/koalaman/shellcheck/wiki/SC2145).

All the tests have passed on bash 3.x locally on my mac.

My bash knowledge quite limited, so I'm not aware of the potential drawbacks for this solution.

Closes #180

[bayandin]

There're some errors from tests on macOS:

./test/test_uninstall.sh: line 65: declare: -A: invalid option
declare: usage: declare [-afFirtx] [-p] [name[=value] ...]
./test/test_uninstall.sh: line 66: 0.9.1: syntax error: invalid arithmetic operator (error token is ".9.1")
./test/test_uninstall.sh: line 67: 0.11.15-oci: syntax error: invalid arithmetic operator (error token is ".11.15-oci")
./test/test_uninstall.sh: line 68: latest: unbound variable

https://travis-ci.com/github/tfutils/tfenv/jobs/326295469

[bayandin]

It looks like all the tests are fine now on default macOS bash

[chenrui333]

@Mike Peachey @Zordrak can you take a look at this PR? Thanks!

[chenrui333]

ping @Zordrak

[Zordrak]

I hate that this is necessary, but I appreciate the effort gone to in doing it.

[bayandin]

Thanks @Zordrak!