fix bash>=4.4 bad substitution

[casperdcl]

• fixes bash>=4.4: bad substitution error on non-word characters #22

locally tested using docker bash:4.4.

Fixes this inconsistency:

x=a/b
echo ${!x}  # bash<=4.3: empty, >=4.4: error bad substitution

There's no real documentation about this change. Vaguely related links:

• https://stackoverflow.com/questions/20615217/bash-bad-substitution
• https://github.com/bminor/bash/blob/bash-4.4-testing/CHANGES

[codecov-commenter]

Codecov Report

Merging #23 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #23   +/-   ##
=======================================
  Coverage   86.84%   86.84%           
=======================================
  Files           1        1           
  Lines          38       38           
=======================================
  Hits           33       33           
  Misses          5        5           

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[casperdcl]

/tag v1.3.2