Bump copyright date to 2018

c.go

doc.go

doc_bls_test.go

doc_test.go

element.go

element_arith.go

element_arith_simple.go

element_fmt.go

element_io.go

element_misc.go

errors.go

generation.go

gmp_big.go

memstream.h

memstream_bsdlike.go

memstream_other.go

memstream_posix.go

pairing.go

params.go

pbc_test.go