depends: Use curl for fetching on Linux

Currently Travis's wget fails fetching qrencode:

Fetching qrencode...
ERROR: no certificate subject alternative name matches
requested host name `fukuchi.org'.
To connect to fukuchi.org insecurely, use `--no-check-certificate'.
OpenSSL: error:14077438:SSL routines:SSL23_GET_SERVER_HELLO:tlsv1 alert internal error
Unable to establish SSL connection.
make: *** [/home/travis/build/luke-jr/bitcoin/depends/sources/download-stamps/.stamp_fetched-qrencode-qrencode-3.4.4.tar.bz2.hash] Error 4