requires "asynctools" now that asynctools was updated

[timotheecour]

/cc @dom96
follows dom96/jester#279
refs nim-lang/RFCs#393 (comment)
refs cheatfate/asynctools#28

[timotheecour]

@Araq could you please tag a release for asynctools to avoid having to do this: #49 (comment) ?

in dom's own words (nim-lang/nimble#890 (comment)):

Specifying a concrete version in a Nimble file has always been a bad idea and largely discouraged. I think that once lock files are merged into Nimble that we should look into disallowing concrete versions in "requires" completely (without lock files this is the only way you can hope to get reproducible builds).

[dom96]

A tagged version won't help much here. I want reproducible builds, but yes, this does run the risk that people's projects will clash with the asynctools dep :/

Maybe a better way is to just run nimble install asynctools in a "before test: ..."