In version 3.x, the output of mongodump is on stderr, but on 2.6.x
it writes information to stdout. This change doesn't break 3.x in
my tests but prevents extraneous junk in beginning of the tar file
when exporting in 2.6.x.
Updated test to match redirect in exec command.
The -o= seems to work fine in 3.x but in 2.x it actually includes the equals sign in the path, resulting in "/=/tmp/tmp.XXXXXX". The mongodump command docs for neither 2.x nor 3.x list the equals sign as part of the -o or --out parameter. It seems to work fine in 3.x with or without the equals sign.
Updated test to reflect change to -o
