📝Add the minimal version requirement on pipenv

[ithinuel]

Description

Add the minimal version requirement on pipenv (>= 2018.11.26).

Test Coverage

• This change is covered by existing or additional automated tests.
• Manual testing has been performed (and evidence provided) as automated testing was not feasible.
• Additional tests are not required for this change (e.g. documentation update).

[codecov]

Codecov Report

Merging #66 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #66   +/-   ##
=======================================
  Coverage   93.55%   93.55%           
=======================================
  Files          39       39           
  Lines        1318     1318           
=======================================
  Hits         1233     1233           
  Misses         85       85           

Impacted Files	Coverage Δ
mbed_devices/_version.py	100.00% <100.00%> (ø)