Removed support for descending index for MYSQL 5.7

[butschster]

@see https://dev.mysql.com/doc/refman/5.7/en/create-index.html

fixed #3

[codecov]

Codecov Report

Merging #4 (3da186b) into master (94acc09) will decrease coverage by 0.05%.
The diff coverage is 75.00%.

@@             Coverage Diff              @@
##             master       #4      +/-   ##
============================================
- Coverage     92.58%   92.52%   -0.06%     
- Complexity     1488     1489       +1     
============================================
  Files            67       67              
  Lines          3841     3841              
============================================
- Hits           3556     3554       -2     
- Misses          285      287       +2     

Impacted Files	Coverage Δ
src/Driver/MySQL/Schema/MySQLTable.php	91.54% <75.00%> (-1.41%)	⬇️
src/Driver/Postgres/Schema/PostgresTable.php	92.77% <0.00%> (-1.21%)	⬇️

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