pom.xml: update ahc to 1.7.20
pom.xml: update ahc to 1.7.20

Signed-off-by: Pierre-Alexandre Meyer <ff019a5748a52b5641624af88a54a2f0e46a9fb5@mouraf.org>

easel/recurly-java-library,7thsense/recurly-java-library,easel/recurly-java-library