Added nil check for tsfd in http_request()

If the opening of the cache timestamp file fails, then
tsfd would be nil.  write is then invoked on it yielding:

  /usr/local/share/lua/5.1/luarocks/fs/lua.lua:592: attempt to index local 'tsfd' (a nil value)

This changeset simply adds the check for nil.  It does no further error
reporting, so luarocks doesn't crash, but it also doesn't report that the
cache was not updated nor that anything out of the ordinary happened.
Merge pull request #233 from neomantra/http_request_err

Added nil check for tsfd in http_request()
Remove repeated entry
Fix issue when source.dir got misdetected if URL ends with character 'c'!
How many years did this go undetected?
Flag -c is running too slow.
handle implict subdirectories when unpacking with LuaZip
use correct path for is_dir check
Merge pull request #235 from siffiejoe/unzip

handle implict subdirectories when unpacking with LuaZip
filter message when deps_mode="none"

On Buildroot, we call `luarocks make --deps-mode=none --keep foo.rockspec`
So, the message about missing dependency is confusing.

```
Warning: skipping dependency checks.
...
Missing dependency for cgilua 5.1.4-1: luafilesystem >= 1.5.0
```
Merge pull request #236 from fperrad/patch-1

filter message when deps_mode="none"
remove exe wrappers, just batch files on windows
check is_lua by compiling the file
Merge pull request #237 from Tieske/no_exe_wrapper

No exe wrapper on Windows
Revert change that broke rocks.moonscript.org — it would skip servers that have no .zip manifest in them.
Removed some unused variables
Merge pull request #239 from mpeterv/remove_unused_variables

Removed some unused variables
Merge branch 'master' of github.com:keplerproject/luarocks
Fix help, as pointed out by @ignacio
Bring the Unix implementation up to par with the Windows one!
Un-module bundled deps