set min similarity to a better default

[monperrus]

when optimizing new hard cases in gumtree-spoon-ast-diff, I realized that the default of min similarity is far too low. A value of 0.5 and 0.6 gives much smaller and cleaner diffs.

WDYT?

(What will CI say? Travis OK, no test breaks.)

[coveralls]

Coverage remained the same at 54.029% when pulling f8e475b on monperrus:fix-sim into f196c52 on GumTreeDiff:develop.

[jrfaller]

Nice! Thanks for the tip, this goes into develop.