Support translations in old WordPress versions

[patrickrobrecht]

• Add load_plugin_textdomain for support of WordPress versions before 4.5
• Declare compatibility with WordPress 4.1+

[codecov-io]

Codecov Report

Merging #91 (cd8297c) into develop (85fe666) will increase coverage by 0.10%.
The diff coverage is 100.00%.

@@              Coverage Diff              @@
##             develop      #91      +/-   ##
=============================================
+ Coverage      40.29%   40.40%   +0.10%     
  Complexity       147      147              
=============================================
  Files              5        5              
  Lines            546      547       +1     
=============================================
+ Hits             220      221       +1     
  Misses           326      326              

Impacted Files	Coverage Δ	Complexity Δ
inc/class-antivirus.php	22.18% <100.00%> (+0.25%)	83.00 <0.00> (ø)

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[stklcode]

• Declare compatibility with WordPress 4.1+

As discussed on Slack, I've successfully tested Antivirus with WP 4.1 (official Docker image). In the meantime I also tested the plugin against a WP 3.8 installation without any issues. Testing requires a custom image, old MySQL version, etc. while 4.1 is quite handy.

So I'm fine with 4.1 for the sake of maintainability.