fix expressionDependencies on expression


 - The value from recursive calls should be returned
 - `fileDependencies` always call `expressionDependencies` with language or symbols instead of            expression, so this chunk is never used and can be safely removed for now.
don't write lock file until snapshot is complete to avoid inconsistency between sources and lock file
collect errors while getting sources for multiple packages, output all errors at the end
remove unused
fetch repository URLs from packrat.lock
bugfix
also fetch sources for dependencies

regression introduced in 6e1790fd9d934f
refactor: extract variable appPackagesFlat
Merge branch 'travis' into bootstrap
need sourcePath
Merge pull request #44 from luckyrandom/patch-1

fix expressionDependencies on expression
Merge pull request #45 from krlmlr/bootstrap

Streamline bootstrapping
Strip whitespace; update .Rproj settings; re-roxygenize
Import pkgVignettes from tools
Only re-set CYGWIN if it's already set
Minor tweak in messaging
In interactive sessions, let the user know they're in a packrat project
Load dependencies from files ending in .r (#48)
Update status to return more information to the user
Search for .R, .Rmd, .Rnw, .Rpres deps