Constrain lbfgs to OCaml < 4.02.0.

lbfgs doesn't build on OCaml 4.02.*:

```
File "src/lbfgs.mli", line 109, characters 24-26:
Error: String literal begins here
Command exited with code 2.
``
tls 0.3.0 and friends
Merge pull request #3266 from Chris00/master

+lbfgs 0.8.6
fix opam version
Merge pull request #3265 from pqwy/tls-0.3.0-etc

tls 0.3.0 and friends
add type_conv as dependency, and tls-0.2.0 requires nocrypto<0.3.0
actually x509-0.2.1 works fine with nocrypto-0.2.0
be more precise about the pieces of mirage we need (thanks @avsm)
Add mirage-console.2.1.0

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Add xentropyd.0.9.1

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Add xe.0.6.3

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #3267 from hannesm/fix-tls-versions-constraints

add type_conv as dependency, and tls-0.2.0 requires nocrypto<0.3.0
xentropy depends on ocaml 4.01.0+ for |>

Signed-off-by: David Scott <dave.scott@citrix.com>
xe.0.6.3 depends on oasis (in the Makefile)

Signed-off-by: David Scott <dave.scott@citrix.com>
containers 0.7.0.1 (missing flags)
xentropyd.0.9.1: improve docs (and retrigger travis)

Signed-off-by: David Scott <dave.scott@citrix.com>
Merge pull request #3262 from yallop/lbfgs-constraint

Constrain lbfgs to OCaml < 4.02.0.
Merge pull request #3268 from djs55/mirage-console.2.1.0

Add mirage-console.2.1.0
Merge pull request #3271 from c-cube/containers.0.7.0.1

containers 0.7.0.1 (missing flags)
Merge pull request #3270 from djs55/xe.0.6.3

Add xe.0.6.3