Remove dependency on dotenv

[jordansissel]

Related: bkeepers/dotenv#457

dotenv did a release that dropped support for older rubies by using syntax that is only valid in newer versions of ruby.

dotenv's parser is used in one place in pleaserun: https://github.com/jordansissel/pleaserun/blob/master/lib/pleaserun/platform/base.rb#L239

At this time, it's unclear how much of dotenv is necessary for pleaserun, and perhaps we could remove this feature or replace its functionality with something built into pleaserun.

This change by dotenv caused some problems for fpm users today: jordansissel/fpm#1918