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

depends/builders/linux.mk

@@ -1,2 +1,2 @@
 build_linux_SHA256SUM = sha256sum
-build_linux_DOWNLOAD = wget --timeout=$(DOWNLOAD_CONNECT_TIMEOUT) --tries=$(DOWNLOAD_RETRIES) -nv -O
+build_linux_DOWNLOAD = curl --connect-timeout $(DOWNLOAD_CONNECT_TIMEOUT) --retry $(DOWNLOAD_RETRIES) -L -o